Recent study results
They discovered that, while both legs of each uninjured runner had very similar hip muscle strength, among the injured runners, the injured side's hip abductor and flexor muscle groups were significantly weaker than the uninjured side.
In addition, the injured side hip adductor muscle group was significantly stronger than the uninjured side. Leg dominance (as determined by preferred kicking leg) did not appear to be a factor in which leg had become injured.
Hip abductor function
The hip abductors move your straightened leg outward from your body on either side, in a plane of motion similar to a sideways step. These muscles are important for stability during the single-leg phase of the running motion. The adductors move your leg back toward midline (in medicine, the imaginary line that divides the right side of your body from the left). The hip flexors and extensors allow front-to-back leg movement, as in running.
Although this study's findings stop short of proving a cause-and-effect relationship, they are the first to show an association between weak muscle strength in areas secondary to the site of pain and the overuse injuries themselves.
The authors point out that lower extremity injury is often multifaceted and therefore runners must consider strength improvements in all muscle groups to not only stave off injury but to rehabilitate after it. The addition of strengthening exercises to specifically identified weak hip muscles may offer better treatment results in patients with running injuries.
(Clin. J. Sport Med., 2005, Vol. 15, No. 1, pp. 14-21)
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